Grails 属于多个域

我有4个类，事件，问题，请求和另一个是附件

每个域看起来都像

    Class Incidents
    {
    // other fields
       static hasOne = [attachment: Attachment]

       static constraints = [attachment nullable:true]
    }

    Class Problems
    {
    // other fields
       static hasOne = [attachment: Attachment]

       static constraints = [attachment nullable:true]
    }

    Class Requests
    {
    // other fields
       static hasOne = [attachment: Attachment]

       static constraints = [attachment nullable:true]
    }

    Class Attachment
    {
    // other fields
       static belongsTo= [
                   incident: Incidents, 
                   problem: Problems,
                   requests: Requests
]

   static constraints = {
        incident nullable: true
        problem nullable: true
        requests nullable: true
}

当我保存事件对象时，它抛出异常，如列'problem_id'不能为null。

---

怎么办？

尝试删除hasOne on类事件、问题、请求，并将其替换为

   Attachment attachment
   static constraints = {attachment: unique: true, nullable:true}       
   static mapping = {
    attachment  cascade: "delete"
    }

---

如果我删除了该关系，那么它是否允许级联删除？对于级联删除，请尝试添加级联映射。我已经更新了答案。